Alternator sizing and external regulator basics

Stock marine alternators (60-120A) are sized for engine start and running loads, not for house-bank charging. If you want to actually recharge a house bank while cruising, upsize to a large-frame alternator (150-250A) with an external smart regulator (Balmar MC-618, Wakespeed WS500, Sterling Alternator to Battery Charger). Rule of thumb: alternator amps should be 25-40 percent of the house bank Ah for AGM, 40-60 percent for lithium (lithium accepts more current). Example: 400Ah AGM bank → 100-160A alternator; 400Ah lithium → 160-240A. Belt: single V-belt tops out around 100A of alternator load, anything larger needs serpentine belt conversion or the belt slips and glazes. Pulley sizing: match alternator pulley to engine pulley for max output at your cruising RPM (usually 1800-2400 RPM engine speed). Temperature sensors: install one on the alternator case AND one on the battery, the regulator uses both to protect the alternator from cooking itself and to accurately compensate charge voltage for battery temperature. Without an alternator temp sensor, high-output alternators fail early from thermal cycling.